1. Field of the Invention
The present invention relates to a lens holding structure, and more specifically relates to a lens holding structure in a lens block of an optical system used in optical devices such as a digital camera, video camera, lens shutter camera and the like.
2. Description of the Related Art
Recent years has seen a demand for compact digital still cameras, lens shutter cameras and the like, and particularly heightened demand for compactness in optical systems that occupy a relatively large space in such equipment. Size of an optical system in a direction of an optical axis thereof, i.e., a distance from a surface of a lens positioned on the outermost object side to the image plane is determined by the optical design specifications and the like. On the other hand, a size of the optical system in a direction perpendicular to the optical axis is normally determined by an outer diameter of a lens element forming a structural element of the optical system. The size of the optical system is determined based on a lens element of the largest outer diameter since each lens element is held by fixedly fitting the outer circumference of the lens element within a holding barrel. Accordingly, in a conventional optical system, the holding barrel cannot be made smaller than the lens element of the largest outer diameter even when there are markedly different outer diameters among the lens elements, thereby disadvantageously enlarging the optical system.
In order to eliminate this disadvantage, U.S. Pat. No. 5,699,200 proposes a lens holding structure that reduces the outer diameter of the holding barrel by providing a notch in the outer circumference of the lens element and securing the lens element by applying an adhesive to the notch. U.S. Pat. No. 5,461,444 proposes a lens holding structure wherein flat surface areas parallel to the optical axis are formed outside effective diameter of a lens element with the lens element fastened to the holding barrel via screws in the flat surface areas.
In the lens holding structure disclosed in U.S. Pat. No. 5,699,200, however, it is difficult to reduce the cost of the lens element due to the complex shape of the lens element provided with the notch, and the notch cannot always be formed depending on the shape of the lens element. Furthermore, in the lens holding structure disclosed in U.S. Pat. No. 5,461,444, sufficient compactness cannot be attained because holding areas formed on the holding barrel for the screw fastening in the flat surface areas have a larger diameter than the largest outer diameter of the lens element.